The present invention relates to a light fired thyristor including a semiconductor body with four zones of alternating conductivity type including a first emitter zone, a control base zone adjacent thereto, a further base zone (main base zone) and a second emitter zone.
When firing of a thyristor is effected by light energy, pairs of charge carriers must be generated in the base zones of the thyristor instead of charge carriers being injected by means of a control contact, and some of the minority charge carriers will then pass through the center pn-junction, which is charged in the reverse direction, and produce a photocurrent. This photocurrent again effects increased injection of the cathode-side emitter and thus a feedback connection for the firing process. Due to the current dependency of the current gain factors in the two partial transistors of the thyristor, a sufficiently high light excitation meets the firing condition that the sum of the current gain factors becomes equal to 1. The light energy is generally introduced through the emitter zone adjacent the control base zone since in this way it is possible to realize localized excitation at high excitation density.
For light fired thyristors sufficient protection must be provided against firing processes or interfering currents, respectively, which are produced as a result of an increase in the forward blocking current due to an increase in temperature or as a result of too high a rate of voltage rise (dv/dt) in the forward direction. These currents are distributed approximately uniformly over the thyristor area in perfect devices and will hereinafter be called interfering currents. This problem is encountered particularly in the firing region of the thyristor. If this region is designed to be sensitive to firing at a low excitation density by suitable geometric dimensions and doping, undesired firing will occur already at low values of interfering currents.